In recent years, in an optical pickup device, there is an advanced tendency toward a shorter wavelength of a laser light source that is used as a light source for reproducing of information recorded on an optical disc and for recording of information on an optical disc. For example, a laser light source with a wavelength of 405 nm such as a violet semiconductor laser or a violet SHG laser conducting wavelength conversion of an infrared semiconductor laser by utilizing generation of second harmonic is coming into practical use.
When these violet laser light sources are used, information of 15-20 GB can be recorded on an optical disc having a diameter of 12 cm in the optical pickup device that uses an objective lens whose numerical aperture (NA) is the same as that of DVD (digital versatile disc), and information of 23-25 GB can be recorded on an optical disc having a diameter of 12 cm in the optical pickup device wherein NA of an objective lens has been enhanced to 0.85. In the present specification, hereafter, an optical disc employing a violet laser light source and a magnetic disc are called “a high density disc” generically.
Incidentally, a value of a product of an optical disc player and/or a recorder is not sufficient, if they can conduct recording and/or reproducing of information properly for this high density optical disc alone. If the present realities that DVD and CD (compact disc) on which various types of information are recorded are available on the market, mere recording and/or reproducing of information is carried out on a high density disc is not sufficient, and an optical disc player and/or a recorder having enhanced product value makes it possible to conduct recording and/or reproducing of information properly in the same way even for DVD and CD owned by a user. From the background of this kind, an optical pickup device installed in an optical disc player and/or a recorder is requested to have capability to record and/or reproduce information properly while keeping interchangeability even for any of three types of optical discs including high density optical disc, DVD and CD.
As a method to realize recording and/or reproducing of information properly while keeping compatibility even for any of a high density optical disc and DVD, further for any of a high density optical disc, DVD and CD, there is considered on object that switches selectively between an optical system for the high density optical disc and an optical system for DVD and CD. However, this object requires a plurality of optical systems, which is disadvantageous for downsizing and causes cost increase.
Therefore, for simplifying the structure of an optical pickup device and for achieving cost reduction, it is preferable to standardize an optical system for a high density optical disc and an optical system for DVD and CD as far as possible to reduce the number of optical parts constituting the optical pickup device as far as possible. Further, standardization of objective optical systems arranged to face optical discs is most advantageous for simplification of the structure of the optical pickup device and for cost reduction.
However, when trying to realize compatibility by using common objective optical elements in the optical pickup device, some contraptions or other are needed for forming a light-converged spot corrected properly in terms of aberration on an information recording surface of an optical disc, because each of a wavelength of a light source and a protective substrate thickness used for each optical disc is different.
As an embodiment for aberration correction, it is considered to change a divergence degree of a light flux that enters an objective optical element. In the embodiment for aberration correction of this kind, off-axis characteristics are worsened (the greater the divergence degree is, the greater the coma generated in the case of lens shift in tracking is) depending on the divergence degree of a light flux entering an objective optical element, which is undesirable and a problem.
Another embodiment for aberration correction is to provide a diffractive structure that gives diffraction effects on an optical surface of an objective optical element (for example, see Patent Document 1).
In the aforesaid conventional technologies, it is possible to correct spherical aberrations properly for all light fluxes having respectively two different wavelengths. However, it is difficult to correct spherical aberrations properly for all light fluxes having respectively three different wavelengths.
In the more specific explanation for the foregoing, wavelengths of light fluxes used respectively for a high density optical disc, DVD and CD are represented respectively by λ1 that is about 400 nm, λ2 that is about 655 nm and λ3 that is about 785 nm, resulting in λ1:λ3≈1:2. Therefore, in the blaze-shaped diffractive structure such as that described in Patent Document 1, a ratio of diffraction order that offers the greatest diffraction efficiency results in λ1:λ3=2:1 (for example, when λ1 is in 6th order, λ3 is in 3rd order). Further, when the ratio of diffraction order of light fluxes for wavelengths λ1 and λ3 is 2:1, a value of λ1×2-λ3×1 gets smaller, because an effect of the diffraction is determined by a difference of wavelength x diffraction order and by a pitch of a diffractive ring-shaped zones. Accordingly, when a diffractive structure is designed with a blaze wavelength to be a value close to a multiple of an even number of wavelength λ1, for example, mutual diffraction effects for a light flux with a wavelength λ1 and a light flux with a wavelength λ3 get smaller, and it becomes difficult to conduct recording and/or reproducing of information respectively by using the same objective optical element for a high density optical disc and CD.
On the other hand, even when a difference of wavelength x diffraction order is small, it is possible to achieve compatibility theoretically by using small diffraction effects. In this case, however, a pitch of a diffractive ring-shaped zones needs to be small, which causes various problems that an amount of light is reduced, manufacturing of lenses becomes difficult and aberrations are greatly generated by wavelength fluctuation within a microscopic range of about several nanometers caused by fluctuation of output of a laser light source.
Further, in the conventional technology of this kind, there still is a problem that an efficiency of using light is poor, even in the case where two optional different wavelengths are used respectively for light fluxes entering objective optical elements. For example, when a diffractive structure is used, and an optical path difference is given so that a substantial phase change may be given to only one of plural light fluxes each having a different wavelength, to generate diffraction of diffracted light of +nth order and diffraction of diffracted light of −nth order from the light flux with another wavelength that has passed through the aforesaid diffractive structure, efficiencies of diffraction for respective diffracted lights are equalized. Since the light flux guided to an information recording surface of the optical information recording medium is only one of the aforesaid diffracted lights, the light intensity is certainly lowered to 50% or less of that of the incident light.
Patent Document 1: Japanese Patent Publication Open to Public Inspection No. 2002-298422